The dispenser cathode which is the typical one among barious cathodes is constituted by impregnating an electron emitting material such as barium aluminate and the like into the pores of a porous base material having high melting point such as tungsten, molybdenum and the like, and such a cathode has a superior electron emitting capability and a longer life expectancy in compared with the conventional ternary-carbonate cathode.
Therefore, such a dispenser cathode can be advantageously applied to a large braun tube or a high resolution braun tube. It has problems, however, that its assembling process and the manufacturing process are such complicated and its electron emitting characteristic is unstable.
FIG. 1 is a partial cross-sectional view of a conventional dispenser cathode. In FIG. 1, a porous metal base material 2, into which an electron emitting material is impregnated, is disposed in a storage tank 3 and the storage tank 3 is attached to a closed top of a cylindrical sleeve 1, where the outside diameter of the storage tank 3 is smaller than that of the cylindrical sleeve 1.
In such a dispenser cathode, since the storage tank 3 and the cylindrical sleeve 1 are overlapped together, thermal transfer during the heating of the heater can not be performed smoothly, thereby lowering the cathode efficiency. Further, when the storage tank 3 which is filled with the same material 2 and the cylindrical sleeve 1 are welded together by a jig, the electron emitting material impregnated into the base material 2 is evaporated or deteriorated by its welding heat, thereby lowering the cathode efficiency.
FIG. 2 illustrates another type of the conventional dispenser cathode. In FIG. 2, the storage tank 3 which is filled with a porous base material 2 such as tungsten is weld-fixed to an open top of cylindrical sleeve 1. In this dispenser cathode, since the storage tank 3 and the sleeve 1 are directly welded together at the edges thereof into an integral body, the electron emitting material impregnated into the base material 2 is evaporated or deteriorated by the welding heat thereby lowering the cathode efficiency. Further, when the storage tank 3 and the sleeve 1 are not contacted substantially such a welding defect can cause a distortion or make a hole.